Well, just because they sent you a plain text password does not mean they don't utilize cryptography when storing your password.
First of all, within cryptography there are hashes and encryption. They should neither encrypt nor store your password in plaintext. They should hash it.
Encryption Function: a two-way function which transforms data in a way which is difficult/near impossible to reverse without knowing the decryption key (I won't bore you about key types).
Hash Function: a one-function which transforms data to a unique signature with which it is seemingly impossible to determine almost anything about the original data (length, text entered, etc.). Hashes are impossible to reverse and can only be discovered by brute-forcing/rainbow table. Brute-forcing is infeasible even with massive amounts of computational power. Rainbow tables can be thwarted if the website uses what we call a "salt" when generating their hash.
So, the website should not even store your password encrypted, but they certainly should not store it in plaintext.
However, what could be happening here is simple. When you register, the script that's called might automatically hash your password, add the hash to the database and, within the same instance, dispatch the email using PHP's
mail() function. By using this function, the plaintext password would never actually get stored within an 'Sent Mail' folder because the
mail() function doesn't use an established email to send from.
Sending the password in plaintext to the user via e-mail is still very insecure, but this doesn't necessarily mean that they don't store it on their end securely.
I know that was long-winded and technical, but the short version is that when you register, they might e-mail it plaintext then, but then store it securely in an irretrievably fashion. It's still a security flaw, but, if they do this, it's not as big of a concern. Just because they send it as an e-mail doesn't necessarily mean it's stored in a 'Sent Mail' folder.